For example, a wheel position detecting device automatically detects a position of a subject wheel in a vehicle. Also, it has been known to use a wheel position detecting device in a tire pressure detecting apparatus for detecting a tire pressure of a wheel. As an example of the tire pressure detecting apparatus, a direct-type tire pressure detecting apparatus has been known.
In the direct-type tire pressure detecting apparatus, a transmitter is directly fixed to each of wheels with a tire. The transmitter is provided with a sensor, such as a pressure sensor. A vehicle body is equipped with an antenna and a receiver. When the transmitter transmits a detection signal of the sensor, the receiver receives the detection signal through the antenna, and detects the tire pressure of the wheel based on the detection signal.
In such a direct-type tire pressure detecting apparatus, data transmitted from the transmitter includes individual identification (ID) information for identifying whether the transmitted data is associated with a subject vehicle to which the tire pressure detecting apparatus is equipped, and for identifying the wheel to which the transmitter transmitting the data is fixed.
To specify the position of the transmitter based on the identification information of the transmitted data, the receiver needs to store beforehand a relationship between the identification information of each transmitter and the position of each wheel. When the positions of the wheels are changed by a tire rotation, it is necessary to register the relationship between the identification information of each transmitter and the position of each wheel again. For example, a patent literature 1 describes a tire pressure monitoring apparatus and a tire pressure monitoring method for automatically registering the relationship between the identification information of each transmitter and the position of each wheel.
In the tire pressure monitoring apparatus of the patent literature 1, a rotational position of each wheel is detected based on an acceleration detection signal of an acceleration sensor of the transmitter. Also, a rotational position of each wheel is detected in a vehicle body when a radio signal is transmitted from the transmitter. The position of the wheel is specified by monitoring a change in a relative angle between the rotational position detected by the transmitter and the rotational position of the wheel detected in the vehicle body.
Namely, the change of the relative angle between the rotational position of the wheel detected in the wheel and the rotational position of the wheel detected in the vehicle body is monitored based on a deviation of predetermined number of data. The position of the wheel is specified by determining whether a variation of the change of the relative angle relative to an initial value is over an acceptable value.
In particular, a wheel speed sensor is provided for each of the wheels. A tooth number of a gear (rotor) is calculated based on a wheel speed pulse outputted from the wheel speed sensor. The wheel position is specified based on a relative angle between the rotational position detected by the transmitter based on the acceleration detection signal of the acceleration sensor and a rotational angle indicated by the tooth number of the gear calculated from the wheel speed pulse.
However, the wheel speed pulse outputted from the wheel speed sensor is likely to largely vary in a low speed region, and it is difficult to properly count the tooth number in the low speed region. Therefore, although the wheel is actually rotating, if the tooth number calculated and the rotational position of the wheel detected based on the acceleration detection signal of the acceleration sensor are largely varied relative to each other, it is difficult to properly specify the position of the wheel.
Also, in the method of the patent literature 1, the wheel position is specified by determining whether a variation is within an allowance range defined by a predetermined allowance value with respect to an initial value. Therefore, in a period where the variation is within the allowance range, the wheel position cannot be specified. Also, since the wheel position is specified based on a standard deviation, a certain amount of data is required. Therefore, the wheel position cannot be specified until the certain amount of data is made. Therefore, it takes time to specify the wheel position.